1. Field of the Invention
This invention relates to information processing and, in particular, information processing done optically.
2. Art Background
Although essentially all information processing is presently done utilizing semiconductor electronics, it has, for a long time, been contemplated to do such processing optically. That is, it has been a desired goal to encode information in the form of an optical image, process this information, record the processed information in a suitable non-linear optical recording medium, and then read the recorded information utilizing a second light source. The processing of the information is accomplished in a variety of ways. For example, the optical information is subjected to optical interactions, i.e., is subjected to a lens or holographic interference. It is also possible to process the information by first manipulating a non-linear optical recording medium, e.g., blocking selected regions in the medium to prevent recording in these areas or illuminating a medium in which information has been previously recorded to modify the recorded information and second, reading the recorded information. (It is possible that the light beam employed for reading also contains information which causes manipulation of the information.) Various recording media have been utilized in the investigation of a suitable optical processing system. The most widely investigated recording media have been photographic films--a recording medium in which information cannot be erased. Greater versatility for processing is obtained, however, with a medium in which recorded information is alterable, i.e., an erasable medium. Widely investigated erasable media involve oxygen-containing crystals such as lithium niobate (LiNbO.sub.2) and bismuth silicate (Bi.sub.12 SiO.sub.20). These crystals are utilized because they both are electrooptic, i.e., they undergo a refractive index change upon application of a field, and photorefractive, i.e., a localized electric field and thus a localized refractive index change are produced by (1) the absortion of light to produce mobile carriers and (2) the separation of the resulting charges. Separation of charge is generally effected in two ways--by relying on inherent drift or diffusion of the carriers or by applying an external field by forming electrodes on the crystal and applying a voltage between these electrodes. The localized refractive index changes are detectable by observing the effect of the crystal on a second incident source of light. The speed at which the refractive index produced in the crystal decays determines the information processing rate. By necessity, the decay to a level of at most 30 percent of the initial value should occur within a time equal to the desired processing rate so that the next bit of information can be processed without unacceptable interference from the previous bit.
One method for processing optical information involves the use of holographic light interactions. In this procedure, a first light source which may or may not carry information is interacted with a second light source carrying information. The resulting interference pattern induces periodic refractive index changes in the crystal which, in turn, produce a diffraction grating. Incident reading electromagnetic radiation is diffracted in these regions. The diffracted light is easily discernible, and the extent of diffraction together with the phase of this diffracted light conveys all the information present in the initial information-carrying light source(s).
In a second processing approach, an image represented by spatial variations in light intensity is made incident on a crystal. The localized incident light produces corresponding localized regions of modified refractive index. When the reading light interacts with these regions, it undergoes a phase change relative to the light that traverses regions having no refractive index modification. This phase modification, and thus the image it represents, is discernible by using, for example, polarizers.
Although materials such as bismuth silicate and lithium niobate do allow recording of information, they generally do not allow information processing at speeds greater than approximately 100 images/sec. The processing rate typically attainable with conventional electronic semiconductor devices is 10.sup.8 bits/sec. Thus for images containing less than 10.sup.6 bits, electronic processors are preferable. Systems have been proposed (although no actual experimentation has been reported) for optical processing of information at extremely high speeds, i.e., speeds greater than 10.sup.12 bits/sec. For example, it has been suggested by A. M. Glass at the Conference on Lasers and Electro-Optics, Baltimore, May 1983, that the use of undoped gallium arsenide having a background impurity concentration of less than 10.sup.15 cm.sup.-3 could perhaps provide processing speeds greater than 10.sup.15 bits/sec. However, continuous wave lasers and other optical equipment that operate at sufficiently high powers for processing speeds faster than 10.sup.12 bits/sec are extremely large and expensive. Thus, the proposed optical systems are either too slow to compete with available semiconductor electronics or are, at present, not economic.